The present invention relates to an air-fuel ratio control apparatus for a multicylinder engine which is designed to feedback control the quantity of fuel supplied in accordance with the detection result of the air-fuel ratio of the engine by an air-fuel ratio sensor and more particularly to a control apparatus which automatically corrects for variation with time of the output characteristic of the air-fuel ratio sensor.
Recently, the air-fuel ratio control of the so-called lean-burn type which makes the air-fuel ratio of an engine as lean as possible has been applied to the engine control of automobiles to simultaneously satisfy the demands for emission control and fuel economy. This method employs an air-fuel ratio sensor capable of substantially linearly detecting the air-fuel ratio over a wide range from the lean to the rich mixture in accordance with both the residual oxygen concentration and unburned gas concentration (hydrocarbons) in the engine exhaust gas.
Then, such air-fuel ratio sensor is used while being exposed into the exhaust gas of a considerably high temperature for a long period of time and therefore its output characteristic changes due to the physical and chemical stresses are not negligible. Thus, if the sensor is used under such condition over a long period of time, errors are increased in the detection results of the air-fuel ratio values and it is difficult to effect the air-fuel ratio control accurately.
Thus, the air-fuel ratio sensor must be used while making corrections for variations of its output characteristic as occasions demand. For example, JP-A-58-57050, filed on Sept. 29, 1981 and laid open on Apr. 5, 1983 in Japan, discloses that during the operation of an engine, all the cylinders are subjected to fuel cut-off so that the exhaust gas become the same in composition with the atmospheric air and the output of the air-fuel ratio sensor is corrected for (calibrated) in a real-time manner. Also, U.S. Pat. No. 4,676,213 of the same inventors (some of them) as the present application, registered on June 30, 1987, discloses an air-fuel ratio control apparatus capable of correcting for variation of the output from an air-fuel ratio sensor.
However, to subject all the cylinders of the engine to fuel cut-off is disadvantageous is that there is the danger of causing the engine to stall in the case of an automobile equipped with an automatic transmission.